Generally, a thermal recording material comprises, on a support, a heat-sensitive recording layer containing, as main components, an electron-donating dye precursor, which is usually colorless or light-colored, and an electron-accepting compound. By application of heat to such a thermal recording material with a thermal head, a thermal stylus, laser beam, etc., an instant reaction between the electron-donating dye precursor and the electron-accepting compound serving as a color developer occurs and thereby a recorded image is produced thereon. Such a thermal recording material is advantageous, for example, in that records can be made thereon with a relatively simple device ensuring easy maintenance and no noise generation. Therefore, thermal recording materials are widely used for a measuring recorder, a facsimile, a printer, a computer terminal, a label printer, a ticket machine for passenger tickets or other tickets, and the like. Particularly in recent years, thermal recording materials are used as receipts of gas, water, electricity and other bill payments, billing statements issued from ATMs at financial institutions, various receipts, public lotteries, thermal recording labels or tags for point of sales (POS) system, etc.
With the diversification of the application of thermal recording materials as set forth above, applications involving print processing have increased. In recent years, strongly desired from the market is a thermal recording material comprising a protective layer that is suitable for print processing in terms of surface strength and has such an excellent solvent barrier property as to prevent color development of the background caused by a solvent for printing, so-called background fogging. More recently, in particular, with the advance of recording systems, thermal recording materials are used in severer conditions. Under conditions such as outdoors and high humidity, thermal recording materials may become wet and stick together (hereinafter, referred to as wet-blocking). To avoid this, a thermal recording material comprising a protective layer with an excellent water resistance is also strongly desired. Further, the frequency of printing on thermal recording materials is increasing, and in a printer with an automatic cutter, powder spill from the coating layers (a heat-sensitive recording layer, a protective layer, etc.) of a thermal recording material upon cutting operation may have a serious effect on, for example, the feeding of the thermal recording material. Therefore, thermal recording materials less prone to powder spill are also desired.
For providing a protective layer of a thermal recording material with a surface strength suitable for print processing and such an excellent solvent barrier property as to prevent color development of the background caused by a solvent for printing, various constitutions containing a modified polyvinyl alcohol resin etc. as a resin for a protective layer and a heat-sensitive recording layer are proposed. For example, Patent Literature 1 describes the use of, as a binder, a random copolymer of a polyvinyl alcohol monomer unit and an ethylene monomer unit in a heat-sensitive recording layer and a protective layer. This method is successful in providing a high surface strength, but the solvent barrier property of the protective layer is insufficient and the water resistance is insufficient. Patent Literature 2 describes the use of, as a binder, PVA having an ethylene unit and a silanol group in a heat-sensitive recording layer and a protective layer. This method is successful in providing an excellent surface strength and solvent barrier property, but the water resistance of the protective layer is poor since the binder has a highly hydrophilic silanol group.
For providing a protective layer of a thermal recording material with an excellent water resistance, various constitutions containing an acrylic resin or a modified polyvinyl alcohol resin as a resin for a protective layer are proposed. For example, Patent Literature 3 describes the use of, as an acrylic resin, a core-shell type aqueous emulsion containing an acrylic copolymer in a core and a (meth)acrylamide copolymer in a shell, and this method is successful in providing the protective layer with a high water resistance. However, since an acrylic resin having a high glass transition point is used for the protective layer of the thermal recording material in pursuit of resistance to a thermal head, the protective layer is naturally hard and fragile. Therefore, such a protective layer is prone to powder spill upon cutting operation, and also unsuitable for print processing in terms of surface strength. As a protective layer containing a modified polyvinyl alcohol resin, for example, Patent Literature 4 and 5 each describe the one containing a water-resistant diacetone-modified polyvinyl alcohol and a crosslinker. These conventional methods are successful in providing the surface of the protective layer with a good water resistance, but the protective layer has an insufficient wet-blocking resistance due to weak adhesion to the underlayer, and is also prone to powder spill upon cutting operation. Patent Literature 6 describes a thermal recording body produced by using, as a modified polyvinyl alcohol, an acetoacetyl-modified polyvinyl alcohol etc. in a protective layer, and adjusting the environmental temperature and moisture. This method is successful in increasing the water resistance, but results in much powder spill and red-yellow tint of the protective layer.
As described above, conventional methods aiming for high surface strength tend to make the coating layer more hydrophilic, and therefore fail to sufficiently improve the water resistance. On the other hand, methods aiming for high water resistance tend to make the coating layer hard and fragile, and therefore fail to sufficiently improve the surface strength and to sufficiently reduce powder spill upon cutting operation.